Dual trench alternating phase shift mask fabrication

ABSTRACT

Fabricating a dual-trench alternating phase shift mask (PSM) is disclosed. A chromium layer over a mask layer, which is over a quartz layer, of the PSM is patterned according to a semiconductor design. The mask layer is dry etched according to deep trenches of a PSM design. The quartz layer is dry etched a first number of times through a first photoresist layer applied over the chromium layer and patterned according to the deep trenches of the PSM design by using backside ultraviolet exposure. The mask layer is dry etched again, according to shallow trenches of the PSM design. The quartz layer is dry etched a second number of times through a second photoresist layer applied over the chromium layer and patterned according to the shallow trenches of the PSM design by using backside ultraviolet exposure.

FIELD OF THE INVENTION

[0001] This invention relates generally to semiconductor devicefabrication, and more particularly to the use of phase shift masks(PSM's) in conjunction with such fabrication.

BACKGROUND OF THE INVENTION

[0002] Since the invention of the integrated circuit (IC), semiconductorchip features have become exponentially smaller and the number oftransistors per device exponentially larger. Advanced IC's with hundredsof millions of transistors at feature sizes of 0.25 micron, 0.18 micron,and less are becoming routine. Improvement in overlay tolerances inphotolithography, and the introduction of new light sources withprogressively shorter wavelengths, have allowed optical steppers tosignificantly reduce the resolution limit for semiconductor fabricationfar beyond one micron. To continue to make chip features smaller, andincrease the transistor density of semiconductor devices, IC's havebegun to be manufactured that have features smaller than thelithographic wavelength.

[0003] Sub-wavelength lithography, however, places large burdens onlithographic processes. Resolution of anything smaller than a wavelengthis generally quite difficult. Pattern fidelity can deteriorate insub-wavelength lithography. The resulting features may deviatesignificantly in size and shape from the ideal pattern drawn by thecircuit designer. For example, as two mask patterns get closer together,diffraction problems occur. At some point, the normal diffraction of theexposure rays start touching, leaving the patterns unresolved in theresist. The blending of the two diffraction patterns into one resultsfrom all the rays being in the same phase. Phase is a term that relatesto the relative positions of a wave's peaks and valleys. In FIG. 1A, thewaves 102 and 104 are in phase, whereas in FIG. 1B, the waves 106 and108 are out of phase.

[0004] One way to prevent the diffraction patterns from affecting twoadjacent mask patterns is to cover one of the openings with atransparent layer that shifts one of the sets of exposing rays out ofphase, which in turn nulls the blending. This is shown in FIGS. 2A and2B. In FIG. 2A, the mask 202 causes an undesirable light intensity asindicated by the line 204. In FIG. 2B, adding the phase shifter 206 tothe mask 202 causes a desirable light intensity as indicated by the line208. This mask 202 in FIG. 2B with the phase shifter 206 added is aphase shift mask (PSM), which is a special type of photomask.

[0005] A typical photomask affects only one of the properties of light,the intensity. Where there is chromium, which is an opaque region, anintensity of zero percent results, whereas where the chromium has beenremoved, such that there is a clear or transparent region, an intensityof substantially 100 percent results. By comparison, a PSM not onlychanges the intensity of the light passing through, but its phase aswell. By changing the phase of the light by 180 degrees in some areas,the PSM takes advantage of how the original light wave adds to the180-degree wave to produce zero intensity as a result of destructiveinterference.

[0006] PSM's have gained increased popularity among manufacturers as thefeature sizes they are tasked with printing become smaller, and thetopography over which these features must be printed becomes morevaried. PSM's offer their customers the opportunity to greatly improvethe resolution capability of their steppers. This allows them to printsmaller feature sizes using the same equipment and processes.

[0007] One particular type of PSM is referred to as an alternating PSM.The PSM of FIG. 2B was one example of an alternating PSM. In analternating PSM, closely spaced apertures are processed so that lightpassing through any particular aperture is 180 degrees out of phase fromthe light passing through adjacent apertures. Any light that spills overinto the dark region from the two edges that are out of phase willdestructively interfere. This reduces the unwanted exposure in thecenter dark region.

[0008]FIG. 3 shows another example of an alternating PSM, and morespecifically, a dual-trench alternating PSM 300. The PSM 300 includestwo layers, a chromium layer 302, and a quartz layer 304. The chromiumlayer 302 is the same type of layer typically found in other, non-PSMphotomasks, in which light is exposed therethrough to an underlyingsemiconductor wafer. Clear regions within the chromium layer 302 allowlight to pass through, whereas opaque regions within the chromium layer302 prevent light from passing through. The clear and opaque regions arearranged to correspond to a desired semiconductor design, or pattern. Inthe PSM 300, there are clear regions 306A, 306B, 306C, 306D, and 306E.

[0009] The quartz layer 304 is more generally a clear or transparentlayer, in which different-sized trenches are alternatively added beneaththe clear regions of the chromium layer 302 to phase shift light passingthrough these clear regions. For instance, the alternating clear regions306A, 306C, and 306E of the chromium layer 302 have shallow trenchesbeneath them in the quartz layer 304. Conversely, the alternatingregions 306B and 306D of the chromium layer 302 have deep trenchesbeneath them in the quartz layer 304. The PSM 300 is an alternating PSMin that the PSM design repeats on an alternating basis for clear regionsof the chromium layer 302, such that one clear region has a shallowtrench beneath it, whereas the next clear region has a deep trenchbeneath it, and so on. The PSM 300 is a dual-trench alternating PSM inthat there are two trenches that repeat, a shallow trench and a deeptrench.

[0010] The manner by which the PSM 300 of FIG. 3 can be fabricatedaccording to the prior art is summarized with reference to FIGS. 4A-4I.In FIG. 4A, the clear regions within the chromium layer 302 are alreadypresent, by a process of photoresist patterning in which the photoresistis first exposed by e-beam writing and then developed, etching thechromium layer 302, and then stripping the remaining photoresist. A newlayer of photoresist 402 has been added, such as by a coating process.In FIG. 4B, the layer of photoresist 402 is exposed to correspond to thedeep trenches 306B and 306D. The exposure is accomplished by e-beamwriting. In FIG. 4C, exposed areas of the photoresist 402 are developedto remove them, and in FIG. 4D, the quartz layer 304 is dry etched toinitially form the deep trenches 306B and 306D.

[0011] The quartz layer 304 is dry etched for 60 degrees, so that ifthere are defects within the quartz layer 304, only 60 degrees of suchdefects will be present. This amount is minimal, and will not affectprinting of the semiconductor device using the PSM 300. The process ofFIGS. 4A-4D is repeated for a total of three times, so that a total of180 degrees of phase shift is achieved within the quartz layer 304.

[0012] In FIG. 4E, another layer of photoresist 402 is added, such as bya coating process. In FIG. 4F, the layer of photoresist 402 is exposedto correspond to all the trenches 306A, 306B, 306C, 306D, and 306E. Theexposure is accomplished by e-beam writing. In FIG. 4G, exposed areas ofthe photoresist 402 are developed to remove them, and in FIG. 4H, thequartz layer 304 is dry etched to completely form the deep trenches 306Band 306D that were previously initially formed, as well as to completelyform the shallow trenches 306A, 306C, and 306E.

[0013] The quartz layer 304 is again dry etched for 60 degrees, so thatif there are defects within the quartz layer 304, only 60 degrees ofsuch defects will be present. The process of FIGS. 4E-4H is repeated fora total of four times. Once the fourth time is finished, the remainingphotoresist 402 is removed, such that the final PSM 300 remains, as hasalready been shown in FIG. 3.

[0014] This conventional approach to manufacturing the dual-trenchalternating PSM 300 has several disadvantages, however. Overlay errorscan error between successive exposures of the photoresist, such asbetween successive e-beam writings. These overlay errors induceanti-reflective layer loss around the etched regions, which can bedifficult to discover during inspection. Because photoresist is exposedfor a total of eight times, there are eight such successive e-beamwritings, and the potential for overlay error inducing anti-reflectivelayer loss is great. Furthermore, e-beam writing in particular can be atime-consuming and/or costly process, such that fabricating the PSM 300will be a lengthy and/or costly process since e-beam writing is usedeight times.

[0015] Therefore, there is a need for a process for fabricating adual-trench alternating PSM that overcomes the problems associated withmanufacturing such PSM's in the prior art. Such a process shouldminimize the use of e-beam writers as much as possible. Minimizing theuse of e-beam writers will allow such a process to avoid as much aspossible overlay errors inducing anti-reflective layer loss.Furthermore, such minimization will decrease the time and/or the cost infabricating dual-trench alternating PSM's. For these and other reasons,there is a need for the present invention.

SUMMARY OF THE INVENTION

[0016] The invention relates to fabricating a dual-trench alternatingphase shift mask (PSM). An opaque layer over a mask layer, which isitself over a transparent layer, of the PSM is patterned according to asemiconductor design. The opaque layer may be a chromium layer, whereasthe transparent layer may be a quartz layer. The mask layer is dryetched according to deep trenches of an alternating PSM design to removethe mask layer therefrom. The transparent layer is dry etched a firstnumber of times through a first photoresist layer applied over theopaque layer and patterned according to the deep trenches of thealternating PSM design by using backside ultraviolet exposure. Thisinitially forms deep trenches of the PSM. The mask layer is dry etchedagain, according to shallow trenches of the alternating PSM design toremove the mask layer therefrom. The transparent layer is dry etched asecond number of times through a second photoresist layer applied overthe opaque layer and patterned according to the shallow trenches of thealternating PSM design by using backside ultraviolet exposure. Thiscompletely forms shallow trenches and the deep trenches of the PSM. Thesecond photoresist layer is then removed.

[0017] The invention provides for advantages not found within the priorart. E-beam writing is significantly reduced in fabricating adual-trench alternating PSM, as compared to the prior art, by insteadusing backside ultraviolet exposure. This significantly decreasedoverlay error, which means that induced anti-reflective layer loss isalso significantly reduced. Backside ultraviolet exposure can be usedbecause of the presence of the mask layer, which may be molybdenumsilicon oxide (MoSiO), which prevents such exposure from affecting thechromium layer. The reduction in use of e-beam writing also means thatthe invention generates a dual-trench alternating PSM in a less costlyand/or less time-consuming manner as compared to the prior art. Otheradvantages, embodiments, and aspects of the invention will becomeapparent by reading the detailed description that follows, and byreferencing the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIGS. 1A and 1B are diagrams showing the difference betweenin-phase and out-of-phase waves.

[0019]FIGS. 2A and 2B are diagrams showing the difference in theresulting light intensity between a photomask without phase shift and aphotomask with phase shift.

[0020]FIG. 3 is a diagram showing an example of an alternatingdual-trench phase shift mask (PSM).

[0021]FIGS. 4A-4H are diagrams showing how the PSM of FIG. 3 can beconventionally manufactured according to the prior art.

[0022]FIGS. 5A and 5B are flowcharts of a method to fabricate analternating dual-trench PSM according to an embodiment of the invention.

[0023]FIGS. 6A-6V are diagrams showing illustratively the method of FIG.5.

DETAILED DESCRIPTION OF THE INVENTION

[0024] In the following detailed description of exemplary embodiments ofthe invention, reference is made to the accompanying drawings that forma part hereof, and in which is shown by way of illustration specificexemplary embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilized,and logical, mechanical, and other changes may be made without departingfrom the spirit or scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

[0025]FIGS. 5A and 5B show a method 500 to construct a dual-trenchalternating phase shift mask (PSM) according to an embodiment of theinvention. In FIG. 5A, a photoresist layer over a chromium layer isfirst exposed according to a semiconductor design (502), such as bye-beam writing. The design is so that the PSM can ultimately be used tomanufacture semiconductor devices of this design, by, for instance,photolithographic processes that use the PSM as the photomask. Thephotoresist layer is developed (504) to remove the exposed parts of thephotoresist layer, and the chromium layer is etched (506) so that thechromium layer is itself patterned according to the semiconductordesign. There is a mask layer beneath the chromium layer separating thechromium layer from a quartz layer. The photoresist layer is thenremoved (508).

[0026] Performance of 502, 504, 506, and 508 is shown illustrativelywith reference to FIGS. 6A-6E. In FIG. 6A, the PSM 600 begins as aquartz layer 602, with a hard mask layer 604 over the quartz layer 602,a chromium layer 606 over the hard mask layer 604, and a photoresistlayer 608 over the chromium layer 606. The quartz layer 602 is generallya type of transparent or clear layer. The hard mask layer 604 may bemolybdenum silicon oxide (MoSiO), or alternatively a differentconductive etching stop layer. The chromium layer 606 is generally atype of opaque layer.

[0027] In FIG. 6B, the photoresist layer 608 is exposed according to thesemiconductor design, as indicated by the regions 610. Exposure may beaccomplished by e-beam or laser writing the semiconductor design in thephotoresist layer 608, to result in the regions 610. In FIG. 6C, thephotoresist is developed to remove those parts of the photoresist layer608 that were exposed or written on, such that there is no longerphotoresist within the regions 610. In FIG. 6D, the chromium layer 606is etched down to the hard mask layer 604, to remove those parts of thechromium layer 606 that were exposed by exposure and development of thephotoresist layer 608. Finally, the photoresist layer 608 is removed,such as by stripping, resulting in FIG. 6E, where the patterned chromiumlayer 606 remains over the layers 604 and 602.

[0028] Referring back to FIG. 5A, a new photoresist layer is nextapplied over the chromium layer (510). This can be accomplished byphotoresist coating the chromium layer. The new photoresist layer isexposed according to deep trenches an alternating PSM design (512), andthe photoresist layer is developed to remove the exposed parts of thelayer (514). The alternating PSM design is so that a dual-trenchalternating PSM results for the semiconductor design when the PSM isfinished being fabricated. The mask layer is next dry etched (516),which removes the mask layer as exposed through the removed photoresistlayer, such that the mask layer has been patterned according to the deeptrenches of the alternating PSM design.

[0029] Performance of 510, 512, 514, and 516 is shown illustrativelywith reference to FIGS. 6F-6I. In FIG. 6F, the new photoresist layer 612has been applied over the chromium layer 606. In FIG. 6G, the regions614 result from exposure of the new photoresist layer 612, such as bye-beam or laser beam writing the alternating PSM design on the newphotoresist layer 612. The underlying quartz layer 602 is not damaged bye-beam writing in particular, because of the presence of the mask layer604. In FIG. 6H, the new photoresist layer 612 has been developed, suchthat the parts of the layer 612 that were written on or otherwiseexposed are removed. The resulting photoresist layer 612 corresponds tothe deep trenches of the alternating PSM design. In FIG. 6I, dry etchinghas been performed through the hard mask layer 604, resulting in removalof those parts of the hard mask layer 604 that were exposed, such thatthe hard mask layer 604 is patterned according to the deep trenches ofthe alternating PSM design.

[0030] Referring back to FIG. 5A, another photoresist layer is appliedover the chromium layer (518). This can be accomplished by photoresistcoating the chromium layer. The photoresist layer is backside exposedthrough the quartz layer (520), such as by using ultraviolet rays. Thebackside exposure reaches the photoresist layer because of thepreviously removed parts of the hard mask layer during dry etching. Thephotoresist layer is developed to remove the exposed parts of thephotoresist layer (522), and the quartz layer is dry etched to initiallyform deep trenches of the PSM (524). The dry etching is performedpreferably at 60 degrees, and 518, 520, 522, and 524 are repeated anumber of times, such as three times, each at 60 degrees. This reducesdefects in the quartz layer from affecting the ultimate printing of asemiconductor wafer using the PSM to fabricate a semiconductor device onthe wafer, since any defect will only exist at 60 degrees, which isminimal enough not to affect the printing.

[0031] Performance of 518, 520, 522, and 524 is shown illustrativelywith reference to FIGS. 6J-6M. In FIG. 6J, another photoresist layer 615has been applied over the chromium layer 606. In FIG. 6K, thephotoresist layer 615 is exposed by backside exposure using ultraviolet,x-, or other types of rays 618. The exposure results in the regions 616through the removed parts of the hard mask layer 604. Using backsideexposure, instead of e-beam or laser beam writing, avoids the overlayerrors that can result from beam writing, such that overlay-inducedanti-reflective layer losses are also avoided. In FIG. 6L, thephotoresist layer 615 has been developed, such that the parts of thelayer 615 that were exposed are removed. The resulting photoresist layer615 corresponds to the deep trenches of the alternating PSM design. InFIG. 6M, dry etching has been performed through the quartz layer 602, toinitially form the deep trenches 620 and 622 of the PSM 600.

[0032] Referring to FIG. 5B, a new photoresist layer is next appliedover the chromium layer (526). This can be accomplished by photoresistcoating the chromium layer. The photoresist layer in particular fillsthe deep trenches that have been initially formed. Next, the photoresistlayer is etched back (528), so that the only photoresist that remains isin the deep trenches. The mask layer is dry etched to remove exposedparts of the mask layer that correspond to the shallow trenches of thealternating PSM design (530). That is, the exposed parts of the masklayer that remain are those that correspond to the shallow trenches ofthe alternating PSM design, since the parts of the mask layer thatcorrespond to the deep trenches of the alternating PSM design havealready been dry etched away. The remaining photoresist that fills theinitially formed deep trenches is then optionally removed, such as byphotoresist stripping (532). Alternatively, rather than performing 526,528, 530, and 532 as have been described, a negative photoresist coatingmay be applied, with backside exposure and subsequent dry etching andoptional photoresist stripping, to achieve the same results.

[0033] Performance of 526, 528, 530, and 532 is shown illustrativelywith reference to FIGS. 6N-6Q. In FIG. 6N, the new photoresist layer 624has been applied over the chromium layer 606, filling the trenches 620and 622 that have been initially formed. In FIG. 6O, the photoresistlayer 624 is etched back so that the only photoresist that remains is inthe trenches 620 and 622 that have been initially formed. In FIG. 6P,dry etching has been performed through the hard mask layer 604,resulting in removal of those parts of the hard mask layer 604 that wereexposed, such that the hard mask layer 604 as patterned now correspondscompletely to the alternating PSM design. However, the only parts of thelayer 604 most recently removed as shown in FIG. 6P correspond to theshallow trenches of the alternating PSM design, since those parts of thelayer 604 that correspond to the deep trenches of the design wereearlier removed. The presence of the photoresist in the deep trenches620 and 622 initially formed protects the trenches from the etchant usedto remove parts of the hard mask layer 604. Finally, in FIG. 6Q, thephotoresist that was in the deep trenches 620 and 622 in FIG. 6P isremoved, such as via stripping.

[0034] Referring back to FIG. 5B, another photoresist layer is appliedover the chromium layer (534). This can be accomplished by photoresistcoating the chromium layer. The photoresist layer is backside exposedthrough the quartz layer (536), such as by using ultraviolet rays. Thebackside exposure reaches the photoresist layer because of thepreviously removed parts of the hard mask layer during dry etchings. Thephotoresist layer is developed to remove the exposed parts of thephotoresist layer (538), and the quartz layer is dry etched tocompletely form shallow trenches and the deep trenches of the PSM (540).The dry etching is performed preferably at 60 degrees, and 534, 536,538, and 540 are repeated a number of times, such as four times, each at60 degrees. This reduces defects in the quartz layer from affecting theultimate printing of a wafer using the PSM to fabricate a device on thewafer, since any defect will only exist at 60 degrees, which is minimalenough not to affect the printing.

[0035] Performance of 534, 536, 538, and 540 is shown illustrativelywith reference to FIGS. 6R-6U. In FIG. 6R, another photoresist layer 626has been applied over the chromium layer 606. In FIG. 6S, thephotoresist layer 626 is exposed by backside exposure using ultraviolet,x-, or other types of rays 628. The exposure results in the regions 630through the removed parts of the hard mask layer 604. Using backsideexposure, instead of e-beam or laser beam writing, avoids the overlayerrors that can result from beam writing, such that overlay-inducedanti-reflective layer losses are avoided. In FIG. 6T, the photoresistlayer 630 has been developed, such that the parts of the layer 630 thatwere exposed are removed. The resulting photoresist layer 630corresponds to both the shallow trenches and the deep trenches of thealternating PSM design. That is, the resulting photoresist layer 630corresponds to the alternating PSM design. In FIG. 6U, dry etching hasbeen performed through the quartz layer 602. This completely forms thedeep trenches 620 and 622 of the PSM 600 that were previously initiallyformed. This also completely forms the shallow trenches 632, 634, and636 of the PSM 600.

[0036] Referring back to FIG. 5B, the photoresist layer that was mostrecently applied is finally removed (542), such as via photoresiststripping. Performance of 542 is shown illustratively with reference toFIG. 6V. In FIG. 6V, the photoresist layer 626 of FIG. 6U has beenremoved. The resulting PSM 600 in FIG. 6V has deep trenches 620 and 622,and shallow trenches 632, 634, and 636. Thus, the PSM 600 is adual-trench alternating PSM.

[0037] It is noted that, although specific embodiments have beenillustrated and described herein, it will be appreciated by those ofordinary skill in the art that any arrangement is calculated to achievethe same purpose may be substituted for the specific embodiments shown.This application is intended to cover any adaptations or variations ofthe present invention. Therefore, it is manifestly intended that thisinvention be limited only by the claims and equivalents thereof.

What is claimed is:
 1. A method for fabricating a dual-trenchalternating phase shift mask (PSM) comprising: patterning an opaquelayer over a mask layer over a transparent layer of the PSM according toa semiconductor design; first dry etching the mask layer according todeep trenches of an alternating PSM design to remove the mask layer fromthe deep trenches of the alternating PSM design; first dry etching afirst plurality of times the transparent layer through a firstphotoresist layer applied over the opaque layer and patterned accordingto the deep trenches of the alternating PSM design by using backsideexposure, to initially form deep trenches of the PSM; second dry etchingthe mask layer according to shallow trenches of the alternating PSMdesign to remove the mask layer from the shallow trenches of thealternating PSM design; and, second dry etching a second plurality oftimes the transparent layer through a second photoresist layer appliedover the opaque layer and patterned according to the shallow trenchesand the deep trenches of the alternating PSM design by using backsideexposure, to completely form shallow trenches and the deep trenches ofthe PSM.
 2. The method of claim 1, wherein patterning the opaque layercomprises: applying a photoresist layer over the opaque layer; exposingthe photoresist layer according to the semiconductor design; developingthe photoresist layer to remove exposed parts of the photoresist layer;etching the opaque layer through the exposed parts of the photoresistlayer; and, removing the photoresist layer remaining.
 3. The method ofclaim 2, wherein exposing the photoresist layer according to thesemiconductor design comprises e-beam writing the semiconductor designon the photoresist layer.
 4. The method of claim 1, wherein first dryetching the mask layer comprises: applying a photoresist layer over theopaque layer; exposing the photoresist layer according to the deeptrenches of the alternating PSM design; developing the photoresist layerto remove exposed parts of the photoresist layer; and, dry etching themask layer through the exposed parts of the photoresist layer.
 5. Themethod of claim 4, wherein exposing the photoresist layer according tothe deep trenches of the alternating PSM design comprises e-beam writingthe deep trenches of the alternating PSM design on the photoresistlayer.
 6. The method of claim 1, wherein first dry etching the firstplurality of times the transparent layer comprises, for each of thefirst plurality of times: applying the first photoresist layer over theopaque layer; backside ultraviolet exposing the first photoresist layer;developing the first photoresist layer to remove exposed parts of thefirst photoresist layer; and, dry etching the transparent layer throughthe exposed parts of the first photoresist layer to initially form thedeep trenches of the PSM.
 7. The method of claim 1, wherein first dryetching the first plurality of times the transparent layer comprisesfirst dry etching the transparent layer sixty degrees for each of thefirst plurality of times.
 8. The method of claim 1, wherein second dryetching the mask layer comprises: applying a photoresist layer over theopaque layer, such that the deep trenches of the PSM that have beeninitially formed are filled; etching back the photoresist layer so thatonly the photoresist layer in the deep trenches remains; dry etching themask layer to remove the mask layer from the shallow trenches of thealternating PSM design; and, removing the photoresist layer remaining inthe deep trenches.
 9. The method of claim 1, wherein second dry etchingthe second plurality of times the transparent layer comprises, for eachof the second plurality of times: applying the second photoresist layerover the opaque layer; backside ultraviolet exposing the secondphotoresist layer; developing the second photoresist layer to removeexposed parts of the second photoresist layer; and, dry etching thetransparent layer through the exposed parts of the second photoresistlayer to completely form the shallow trenches and the deep trenches ofthe PSM.
 10. The method of claim 9, further comprising removing thesecond photoresist layer.
 11. The method of claim 1, wherein second dryetching the second plurality of times the transparent layer comprisessecond dry etching the transparent layer sixty degrees for each of thesecond plurality of times.
 12. The method of claim 1, wherein the opaquelayer comprises a chromium layer.
 13. The method of claim 1, wherein thetransparent layer comprises a quartz layer.
 14. The method of claim 1,wherein the mask layer comprises a molybdenum silicon oxide (MoSiO)layer.
 15. A semiconductor device fabricated at least in part by using adual-trench alternating phase shift mask (PSM) fabricated at least inpart by performing a method comprising: patterning an chromium layerover a mask layer over a quartz layer of the PSM according to asemiconductor design; first dry etching the mask layer according to deeptrenches of an alternating PSM design to remove the mask layer from thedeep trenches of the alternating PSM design; first dry etching a firstplurality of times the quartz layer through a first photoresist layerapplied over the chromium layer and patterned according to the deeptrenches of the alternating PSM design by using backside exposure, toinitially form deep trenches of the PSM; second dry etching the masklayer according to shallow trenches of the alternating PSM design toremove the mask layer from the shallow trenches of the alternating PSMdesign; and, second dry etching a second plurality of times the quartzlayer through a second photoresist layer applied over the chromium layerand patterned according to the shallow trenches and the deep trenches ofthe alternating PSM design by using backside exposure, to completelyform shallow trenches and the deep trenches of the PSM.
 16. The deviceof claim 15, wherein first dry etching the first plurality of times thequartz layer comprises, for each of the first plurality of times:applying the first photoresist layer over the chromium layer; backsideultraviolet exposing the first photoresist layer; developing the firstphotoresist layer to remove exposed parts of the first photoresistlayer; and, dry etching the quartz layer through the exposed parts ofthe first photoresist layer to initially form the deep trenches of thePSM.
 17. The device of claim 15, wherein first dry etching the firstplurality of times the quartz layer comprises first dry etching thequartz layer sixty degrees for each of the first plurality of times. 18.The device of claim 15, wherein second dry etching the second pluralityof times the quartz layer comprises, for each of the second plurality oftimes: applying the second photoresist layer over the chromium layer;backside ultraviolet exposing the second photoresist layer; developingthe second photoresist layer to remove exposed parts of the secondphotoresist layer; and, dry etching the quartz layer through the exposedparts of the second photoresist layer to completely form the shallowtrenches and the deep trenches of the PSM.
 19. The device of claim 15,wherein second dry etching the second plurality of times the quartzlayer comprises second dry etching the quartz layer sixty degrees foreach of the second plurality of times.
 20. A dual-trench alternatingphase shift mask (PSM) fabricated at least in part by performing amethod comprising: patterning an chromium layer over a mask layer over aquartz layer of the PSM according to a semiconductor design; first dryetching the mask layer according to deep trenches of an alternating PSMdesign to remove the mask layer from the deep trenches of thealternating PSM design; for each of a first plurality of times, applyinga first photoresist layer over the chromium layer; backside exposing thefirst photoresist layer; developing the first photoresist layer toremove exposed parts of the first photoresist layer; dry etching thequartz layer through the exposed parts of the first photoresist layer toinitially form deep trenches of the PSM; second dry etching the masklayer according to shallow trenches of the alternating PSM design toremove the mask layer from the shallow trenches of the alternating PSMdesign; for each of a second plurality of times, applying a secondphotoresist layer over the chromium layer; backside exposing the secondphotoresist layer; developing the second photoresist layer to removeexposed parts of the second photoresist layer; dry etching the quartzlayer through the exposed parts of the second photoresist layer tocompletely form shallow trenches and the deep trenches of the PSM; and,removing the second photoresist layer.